In the traditional cellular communication technologies, voice, data and other traffic between two UEs are exchanged through base stations where they reside, and a core network.
Device to Device (D2D), which is a direct communication technology, refers to that data can be transmitted between adjacent UEs in a short-distance range over a direct link without being forwarded by a central node (i.e., base station).
The Long Term Evolution (LTE) D2D technology refers to D2D communication controlled by an LTE network in an LTE licensed frequency band. The LTE D2D characteristic has been introduced to enable the LTE technology to evolve from the simple radio mobile cellular communication technology toward the universal connectivity technology.
If D2D communication is introduced to the LTE licensed frequency band, then radio resources will be shared by the D2D communication link and cellular communication, which is also a fundamental issue in integrating the D2D communication technology into the cellular system.
The radio resource can be multiplexed for sharing. Take the uplink radio resources in an LTE Frequency Division Duplex (FDD) system being multiplexed as an example, a receiver of D2D communication will receive a radio signal in a sub-frame allocated for D2D communication (simply a D2D sub-frame); and the receiver of D2D communication may still need to transmit a cellular radio signal in a sub-frame adjacent to the sub-frame (the sub-frame in which the cellular radio signal can be transmitted referred to as a cellular communication sub-frame).
If radio resources in the LTE FDD system are multiplexed for D2D communication, then an LTE uplink or downlink pilot structure may be used in a D2D sub-frame.
The fourth Orthogonal Frequency Division Multiplex (OFDM) symbol in each time slot is occupied by a demodulation pilot signal transmitted in the uplink in the LTE system. For a normal Cyclic Prefix (CP), the fourth OFDM symbol and the eleventh OFDM symbol in one sub-frame are pilot symbols; and for an extended CP, the fourth OFDM symbol and the tenth OFDM symbol in one sub-frame are pilot symbols.
For a normal CP, six Resource Elements (REs) in each of the last two OFDM symbols in each time slot are occupied by an LTE downlink demodulation pilot signal.
If the resources in the LTE FDD system are multiplexed for D2D communication, then there will be a significant overhead of resources for the pilot signal in either the LTE uplink or downlink pilot structure.